Ancient armored amoebas called fusulinids grew so large that they were clearly visible to the naked eye.

Imagine a world that swarmed with armor-clad single-celled organisms so large and monstrous that you could see them with the naked eye, and even pick them up and feel them wriggling in your hand.

It might sound like an alien world, but 300 million years ago giant amoebas fitting this description swam the seas right here on Earth.

Scientists have long been unable to prove how these ancient protozoa, called fusulinids, were able to grow to lengths as long as 10 centimeters-- a massive size for a single-celled creature. But now a research team headed by Jonathan Payne, a paleobiologist at Stanford University, has collected new evidence that may finally put the mystery to rest, according to ScienceNews.org.

One reason that single-celled organisms don't grow to such epic sizes today is because they are limited by how far oxygen can penetrate into them. But 300 million years ago, during the Paleozoic Era, the atmosphere was much different; Oxygen levels were so high that the air could almost spontaneously combust.

Payne and his colleagues thus looked for hints that heightened levels of oxygen might explain the amoebas' size. He enlisted undergraduates and high school students to compare how the armor-like shells that once adorned these ancient microorganisms changed over geological time. Sure enough, they found that the amoebas tended to grow larger during periods when atmospheric oxygen rose. After oxygen peaked at levels 66 percent higher than today and began to fall, the amoebas shrank accordingly.

The way the amoebas grew over time also revealed the influence of oxygen on their size. Instead of ballooning like beach balls, the amoebas elongated. While they grew to lengths of up to 10 centimeters, they rarely measured more than about two millimeters across. These "stringy" shapes would have been much more efficient than round ones at allowing oxygen to penetrate throughout the cell.

Payne and his team also studied how oxygen levels can influence the size of modern amoebas. They found that species which live close to the surface, where oxygen levels are higher, are typically bigger than those living in deeper waters.

All of this evidence taken together points clearly to oxygen being the spinach that allowed Paleozoic amoebas to grow so large.

Luckily, the odds of ever encountering one of these giant armor-clad amoebas today are pretty slim. Current oxygen levels make such large sizes impossible to achieve. But who knows, someday the Earth may again blossom into an oxygen-rich enclave, and the giant amoebas will live again.